I recently received several questions regarding the proper time to fly after diving. The dilemma usually occurs at the end of a vacation, when you want to make those last dives. Is it reasonable to dive on the day of departure? Should you finish diving the day before or even two days before flying? In 1960, several crew members from a commercial airplane developed decompression sickness when they flew shortly after diving. Since then, many other cases of decompression sickness have been reported in divers who flew shortly after diving. It is known that ascent to altitude immediately after diving causes a significant risk for decompression sickness. A commercial jetliner exposes passengers to pressure equivalent to about 8,000 feet of altitude. Exposure to a pressure lower than surface pressure after diving increases the tendency for bubbles to form and can cause decompression sickness. The table below shows the relation between altitude and atmospheric pressure.

ALTITUDE/ATMOSPHERIC PRESSURE

Pressure Pressure

Altitude PSI ATM

(Feet) (Absolute) (Absolute)

0 14.7 1.000

4,000 12.7 0.863

8,000 10.9 0.742

12,000 9.3 0.636

16,000 8.0 0.542

Reduction of pressure from one atmosphere will have the same effect as ascending from depth with excess nitrogen in the tissues. Ascent to altitudes above 18,000 feet can cause decompression sickness without diving. Since we are constantly exposed to a pressure of one atmosphere at sea level, the tissues of the body are in equilibrium with the nitrogen partial pressure in the air. When pressure is lowered by going to altitude, the nitrogen becomes supersaturated and bubbles can form. Under ordinary circumstances, going from ground level to altitudes below 18,000 feet (0.5 ATA) will not cause decompression sickness. There is no risk of decompression sickness from skiing on high peaks, driving over mountain passes or flying in a commercial aircraft. Astronauts are at risk for decompression sickness when they perform space walks. The low pressure in the space suit (about 0.4 ATM absolute) is equivalent to a high altitude and, with prolonged exposure, decompression sickness is known to occur. Going from depth to the surface in diving and from ground level to altitude in aviation can produce similar risks of decompression sickness under some circumstances.

The Divers Alert Network recorded 16 cases of decompression sickness that occurred during or after flying, where the divers followed recommended decompression procedures. Two of the subjects developed symptoms three days after flying home from a diving vacation. Some of the cases occurred with a two to seven hour interval between flying and diving. Further analysis suggests that a 12 hour minimum might be needed for safe flying, with extension of this time when diving is extreme.

Experimental studies in animals showed that exposure to altitude after diving increases the severity and incidence of decompression sickness. Theoretical calculations for a dive to the no decompression limit followed by a flight indicate that six to eight hours is adequate for a commercial flight following a single dive. The typical sport diving pattern of multiple dives over several days followed by flying has not been analyzed theoretically. There is concern that three or four dives a day for several days causes an accumulation of nitrogen. This may cause a problem when flying.

Recommended intervals before flying range from 2 to 12 hours for no decompression diving. A reasonable recommendation is that divers who have had less than two hours total no decompression dive time during the 24 hours before flying should wait 12 hours before flying. Divers who do multi-day diving should wait 18 to 24 hours before flying. Bubbles present from a previous dive may not cause problems on the surface but may cause decompression sickness as they expand at altitude. To avoid bubbles, dives should be conservative. You can minimize risk by reducing total diving exposure on the last two days before traveling by aircraft.

In a given year there are likely to be at least 300,000 to 400,000 people who fly home 12 to 24 hours after their last dive. The incidence of decompression sickness among these divers is estimated to be about 0.004 percent. This low incidence indicates current procedures are safe.

Basic guidelines for vacations should include conservative diving, limiting repetitive dives to three per day, doing only no decompression dives that are not at the no decompression limits and reducing total diving time in the last day or two. Flying should be done the day after diving is finished. Even though this may not be a full 24 hours after diving, experience to date indicates this will produce